1. Field of the Invention
This invention relates to a slurry storage tank for holding a slurry, having a suspended sedimentatable solid material, in a homogeneous state.
2. Prior Art
Conventionally, freight trains, trucks, belt conveyers and the like have been used to transport a solid material such as coal, iron ore and non-ferrous metal. Such transportation required much time and labor for loading and unloading. For this reason, a pipeline transportation has now been extensively used. In this transportation method, the solid material in pulverized form is suspended in water to provide a slurry, and the slurry is transported through the pipeline. The slurry is transported to a distant station where a large amount of slurry is stored in storage tanks for a long period of time. Such storage tanks usually have a capacity of 1000 to 50000 tons. It is important that the slurry in the storage tank is maintained in a homogeneous state when the slurry is withdrawn from the storage tank and delivered to a processing installation. In the case where the suspended solid particles become settled to the bottom of the storage tank, the slurry can not be continuously withdrawn from the storage tank so that the supply of the starting material or slurry to the processing installation is interrupted. As a result, the intended process can not be carried out. Even if the slurry can be supplied to the processing installation, the intended process is adversely affected because there is a considerable change in concentration of the slurry.
When the solid material is pulverized to a predetermined particle size and is suspended in water to form a slurry having a predetermined concentration, the suspended particles will interfere with one another so that the sedimentation rate of the suspended solid material is very low. Although the addition of a surface active agent is advantageous in preventing the solid material from settling to the bottom of the storage tank, the use of such a surface active agent is not desirable from an economical point of view.
The speed at which the suspended solid material falls toward the tank bottom is usually adjusted to 0.1 to 1 m per hour. Therefore, in order that the solid material may not be separated from the liquid carrier to such an extent that an interface is formed, and that the concentration of the slurry may not differ from one portion to another, it is necessary to move the slurry upwardly at a speed greater than the speed of falling of the solid material. For example, the speed of falling of the solid material is not more than 1 m per hour, the rate of movement of the slurry should be above 1 m per hour to keep the slurry in a substantially homogeneous state.
One known storage tank is provided with a propeller type agitator for keeping a slurry in motion at a predetermined rate as described above, the agitator being mounted on the side wall of the storage tank. This conventional storage tank has been found not entirely satisfactory, however, in that an adequate agitation can not be achieved in the central portion of the tank where the tank is of a large size. As a result, the solid material tends to deposit on the central portion of the tank bottom. This is true particularly when liquids of high vicosity such as heavy oil are employed.
Another conventional storage tank has a vertically disposed agitator arranged centrally within the storage tank. This conventional storage tank has also been found not wholly satisfactory in that a considerable power is required for operating the agitator. For example, where the agitator is operated by a turbine for agitating a dense slurry, it requires an agitating power of 1 to 2 KW/m.sup.3.
The present inventor has found that a slurry, which contains a solid material of a predetermined particle size and has a predetermined concentration, can be maintained in a homogeneous state by slowly circulating the slurry at a predetermined rate through external pipes. Thus, the slurry can be maintained in a homogeneous state without vigorously agitating the slurry. This circulation is carried out by withdrawing the slurry from the lower portion of the storage tank and pumping back to the surface of the slurry in the storage tank at such a rate that a given portion of the slurry in the tank slowly moves downwardly. This method is suitably carried out even if the storage tank is of a large size so long as the slurry in the storage tank is not subjected to considerable irregular motion. It has also been found that when a certain kind of slurry, for example, such as a slurry prepared by suspending powder coal in oil and containing a surface active agent to stabilize the suspension of the powder coal, is subjected to vigorous agitation, the stabilization of the slurry is rather affected adversely. Thus, in such a case, vigorous agitation is not suitable for preventing the sedimentation.